Disclaimer: This post is meant to provide general knowledge in the area of plant disease. It is not intended as any kind of guide to diagnose or treat disease or other conditions.
Plants moved on to the land about 470 million years ago and during that time there has been a constant struggle between them and microbes looking to take advantage of the energy and nutrients that the plants produced. We do not categorize animals the same way since they eat plant material rather than parasitize it.
Types of Plant Pathogens
The most common and economically detrimental types of plant diseases, causing about 85% of disease. The biology and life cycles of the different species can be very diverse and complex.
Infections generally start out with a spore from an infected plant being carried by the wind to another plant (though not all fungi produce spores). Once it lands on a susceptible plant, the spore germinates and penetrates the cuticle of the plant with its hyphae which are fungal vegetative growth structures. The hyphae can either kill off the plant cells and consume them or tap into them directly to obtain their nutrients. Eventually new spores are formed and the cycle begins anew. In addition some species live in the soil and can infect plants from there.
Fungi not only diminish yield in crops but can also impact humans by producing mycotoxins. These are potentially deadly poisons for humans and animals upon consumption of contaminated plant material.
*Did you know?*
The fungal pathogen, wheat rust can cause losses of yield of up to 20 percent. In the early days of wheat farming on the plains of North America, the settlers brought barberry (Berberis vulgaris) with them from Europe as it was a very useful plant. They didn't realize that wheat rust uses barberry during the second stage in its life cycle and the cultivation of barberry was leading to epidemics of rust. Canada and the United States started programs to eradicate barberry which led to a significant reduction of disease infection.
Also known as 'water moulds' they are generally lumped together with fungi but are not true fungi. They thrive in damp conditions and the spores are able to 'swim' to infect plants. Pythium (root rot) is the best known in this class of organisms.
While extremely common in all environments there are surprisingly few that infect plants. They infect the areas between the cells and cause a wide array of symptoms. Some also produce chemicals that are toxic to the plant or enzymes which break down cellular structures.
*Did you know?*
Agrobacterium is a bacterial plant pathogen that injects its DNA into the plant genome and produces a cancer like growth known as a gall. An infection happened with sweet potato 8000 to 10,000 years ago and the plant was able to survive and reproduce, passing on Agrobacterium genes to subsequent generations. To this day the sweet potato genome carries genes and produces proteins which originated from this infection.
It is debatable whether or not viruses are actually living or not. They're pieces of genetic material surrounded by a protein coat which do not have any metabolic function of their own. When they come in contact with a compatible cell, the DNA or RNA contained in it will hijack the cell, turning it into a virus factory to start the infection anew. They are generally spread through vectors like insects which carry them from an infected plant to a healthy one. The tobacco mosaic virus is among the most studied of any type of virus.
They are microscopic round worms which live in the soil. They can parasitise root systems often forcing the plant to produce nodules to accommodate them.
The least common type of pathogens but possibly the most interesting, represented only by mistletoe and dodder. Their roots burrow into the circulatory systems of the host plants. In the case of mistletoe, it steals water and minerals from the host. Then produces its own energy from photosynthesis. The dodder however steals energy as well and does very little to no photosynthesis.
Methods of Controlling Pathogens
Involves separating infected plants from the healthy population, as well as destroying plants, or less effectively trimming off infected areas.
Genetic diversity within a species will ensure that some individuals will be more able to fend off an attack than others. Plant breeders can select cultivars that have superior resistance to disease and cross it with cultivars having other desirable traits such as yield. Typically, for a new variety to be registered it must meet minimum standards for disease resistance.
Biochemical differences between the host and the pathogen are exploited in order to control the disease. For example fungal cell membranes contain a component called ergosterol which plants don't produce. Myclobutanil is a fungicide which prevents the synthesis of ergosterol. Therefore an application will kill the fungal pathogen but not harm the plant. Please note that we are not advocating the use of Myclobutanil, but are merely using it as an example of how pesticides work.
*Did you know?*
Several of the most popular varieties of wine grapes are hundreds of years old. During that time downy and powdery mildew have evolved to be more virulent. However there has been no genetic changes in the grapes making them extremely susceptible to disease. For that reason wine grapes are heavily dependent on fungicides, typically requiring 12-15 applications per growing season. While modern disease resistant varieties have been developed, consumers are uninterested in moving away from types of wine produced with the older cultivars.
Planting the same crop year after year will build up the population of pathogens. Changing the type of plants grown will either deprive the pathogen of their food source and in some cases can actively kill the pest. For example, nematodes are very susceptible to the toxic properties of glucosinolates (the hot spicy flavour found in mustard and horseradish) and so adding mustard to a crop rotation will help to keep their numbers in check.
Disease is a major part of horticulture / agriculture, from a minuscule spot on a leaf, up to the Irish potato famine which killed or displaced millions of people. Ultimately it's far easier to use methods to prevent pathogens from getting established than it is to eradicate them once they've been established.